Wikipedia mentions a mostly unused "MSGI" for "minority sexual and gender identities", but I'm not sure people appreciate the "minority" part.
I think we need to stop beating around the bush here. cis-hetero is
the majority, and will be for the forseeable future until we achieve transhumanism ala The Culture
or something. Not better, but yes, more numerous and yes, "standard" or "default" from a purely biological standpoint. If we were to enumerate every alternative sexuality (or lifestyle? Since apparently the word "sexuality" is no longer even fit to encompass the concept) things would get ridiculous, and I can only assume that the number is constantly changing anyway since it's the human mind we're talking about. At some point it stops being about rights, and becomes some kind of ego trip about that persons own special kind of "differentness", which only hinders acceptance from others.
I think the biggest reason Ts are left out is precisely because they haven't gained mainstream acceptance yet. I would say at this point people are comfortable enough with LGBs that, excluding some bigots, they can integrate smoothly into most any workplace (disclaimer: I am not gay, but I think this is a pretty fair statement for my generation, which comprises the majority of the military).
Ts on the other hand make a lot more people uncomfortable, including me. One reason is because they are so few in number, and are little-understood. Another reason is that their fight for rights hasn't been in the public light as long as the GLBs' has. The last reason is because of the physical aspect that isn't present with GLBs. Until Ts become more socially accepted, attempting to integrate them into the military would likely cause more problems from a human relations standpoint than it would be worth. The military is primarily for protecting our country, not making human rights statements, as cold as that may sound.